Frequency standard using an atomic fountain of optically trapped atoms

Abstract

Beams of laser light trap and cool cesium atoms in a small vapor cell and put the atoms in a particular quantum mechanical state. The lasers are then configured so as to launch the atoms upward by shifting the frequencies of the vertically propagating lasers. The atoms pass through a microwave waveguide during both their ascent and descent. The microwave field is applied briefly each time the atoms are in the center of the waveguide so that the microwaves excite the cesium "clock" transition. Once the atoms have fallen back to where they started, the laser fields are turned on in a particular sequence. The fraction of the atoms that make a quantum mechanical transition is measured by observing the laser light scattered by the atoms. That signal indicates how close the microwave frequency is to the atomic transition. The laser cooling reduces the relative motion of the atoms so that the atoms can be observed longer. The resulting atomic resonance measured is much narrower.

What is claimed is: 
     
       1. A precision frequency standard, comprising: means for optically trapping atoms from a vapor;   means for ejecting the optically trapped atoms from said optical trapping means;   means for exciting the atoms, the excitation means including a microwave oscillator;   means for measuring a fraction of atoms exited by the excitation means;   means for comparing the fraction of excited atoms with atoms that have not been excited;   means for adjusting the microwave oscillator to maximize the fraction of atoms being excited; and   means for magnetically guiding the atoms between the exciting means and the measuring means.   
     
     
       2. A precision frequency standard as claimed in claim 1, wherein the ejecting means comprises means for shifting the frequency of the light in the optical trap along one axis. 
     
     
       3. A precision frequency standard as claimed in claim 2, wherein the frequency shifting means comprise two movable mirrors. 
     
     
       4. A precision frequency standard as claimed in claim 1, wherein the optical trap comprises means for directing laser light along six axes. 
     
     
       5. A precision frequency standard as claimed in claim 4, wherein diode lasers produce the laser light. 
     
     
       6. A precision frequency standard as claimed in claim 1, wherein the atoms are atoms of cesium. 
     
     
       7. A precision frequency standard as claimed in claim 1, wherein the atoms are atoms of cesium and the measuring means measures the 6S F=3, m=1 to 6S F=4, m=-1 transition. 
     
     
       8. An optical trap comprising: means for generating a plurality of laser beams;   means for forming a vacuum around a three dimensional target area;   means for forming a vacuum around the optical trap;   means for introducing a predetermined species of atoms into the target area;   means for introducing a predetermined species of atoms into the vacuum;   means for directing the laser beams to the three dimensional target area such that the net light pressure is zero in the target area;   means for changing the frequency of two laser beams in at least one direction so as to make the light pressure in the target area nonuniform in said at least one direction; and   means for magnetically guiding the atoms along a predetermined path.   
     
     
       9. An optical trap as claimed in claim 8, wherein the directing means comprises means for pointing a plurality of laser beams at the target area. 
     
     
       10. An optical trap as claimed in claim 8, wherein the means for generating a plurality of laser beams comprises laser diodes. 
     
     
       11. An optical trap as claimed in claim 8, wherein the frequency changing means comprises two movable mirrors. 
     
     
       12. An optical trap as claimed in claim 8, wherein the vacuum forming means comprises a sealed cell.